1. Field of Invention
The present invention relates to a semiconductor device. More particularly, the present invention relates to a semiconductor transistor for testing, which is compatible with fabrication process of common semiconductor devices and can be achieved without additional processing steps, and thus the deterioration of various internal portions of the device under electrical stress can be conveniently detected.
2. Description of Related Art
In recent years, since flat panel displays have been gradually popularized and have replaced traditional CRT screens, thin-film transistor liquid crystal display (TFT-LCD) related industries are developing rapidly. Among others, the thin-film transistor (TFT), a crucial component, turns out to be one of the most important subjects currently.
It is generally known that the active region of a TFT comprises polysilicon silicon or amorphous silicon rather than single-crystal silicon, thus having a number of defects which lead to many problems, such as reliability. Therefore, it is urgent to improve the reliability of TFTs.
The active region of a TFT, also known as the channel region, is the region below the gate structure. Conventionally, in order to detect the deterioration of the channel region under persistent operating voltage, a set of test operating voltages is applied to electrodes of the gate, source, and drain, so as to generate an electrical stress on the transistor. After applying the electrical stress for a period of time, a characteristic curve of the transistor is tested to detect the change of the characteristics of the transistor caused by applying the electrical stress.
However, in addition to the fact that additional processes are required to fabricate the transistor for testing, the conventional detecting method can only test the overall structure of the transistor, and cannot detect various portions of the channel region due to the structure of the transistor for testing. Therefore, the conventional detecting method may cause wrong judgment. One of the possible situations is that, for example, a part of the channel region has been deteriorated. However, as for the overall testing, only a slight deviation occurs, and cannot indicate the existence of deteriorated portion. Thus, the conventional detecting result may lead to a wrong judgment.